Hi Tom, perhaps the contra piston is gummed up and frozen down in the bore. Broken head screws would certainly indicate the piston has made contact with the contra or an hydraulic lock up (incompressable fuel filling the space between the piston crown and the contra piston ). I hope the conrod hasn't bent. Although you're adjusting the comp. screw, the contra piston may not be moving back up the bore when the comp. screw is wound out. I had the same problem with a N.I.B. 40 Y.O. DC Merlin I found on e*a*. It had probably only ever had a brief run at the factory, leaving enough castor inside to "gum up the works "after all those years. Soak the liner in lacquer thinners overnight to help dissolve the old gummed castor around the contra piston , sit the liner on a piece of wood, then using a WOODEN dowel, gently tap the contra back and forwards to free it up. An email to PAW or Eric Clutton ( Dr. Diesel ) should help find out what thread and size the screws are. Just my 2 cents worth. I hope you can get it running.